Apoptosis is an essential cellular process important for many physiological functions. Dysregulation of apoptosis has been found to be involved in multiple diseases including cancers and neurodegenerative diseases. Neurons, after suffering various insults, undergo apoptosis in neurodegenerative diseases. Identification of new proteins mediating apoptosis and elucidation of the underlying mechanisms will be important for developing new strategies to fight against these diseases. An important pathologic feature and the primary cause of Alzheimer's disease (AD) is overproduction/accumulation of ?-amyloid (A?) peptides that form extracellular senile plaques in the brain. A? peptides are derived from ?-amyloid precursor protein (APP) through sequential cleavages by ?-secretase and ?-secretase. Cleavage of APP by ?-secretase also releases the intracellular domain of APP (AICD) which has been suggested to play a role in gene transcription regulation. Moreover, APP, ?-cleaved C-terminal fragment of APP (C99), AICD and smaller fragments (C31 and Jcasp), derived from AICD by caspase cleavage, have been shown to be cytotoxic when overexpressed, suggesting their involvement in apoptosis. In our preliminary studies, we have identified an APP/AICD-interacting protein, appoptosin (also known as SLC25A38), which belongs to the mitochondrial carrier protein (MCP) family. Importantly, we find that appoptosin is a pro-apoptotic protein and its overexpression can induce caspase-dependent apoptosis, for which APP is partially required. In addition, the level of appoptosin is elevated in AD brains and in neurons upon insult treatments, whereas downregulation of appoptosin can protect neurons against neurotoxicity. Therefore, we hypothesize that appoptosin-mediated apoptosis plays an important role in neurotoxicity-induced neurodegenerative diseases such as AD. In this application, we will further study the molecular pathways underlying the apoptosis induced by appoptosin and identify the biological function of appoptosin. We will ascertain whether and how APP and its metabolites affect appoptosin and reciprocally, study whether appoptosin regulates APP processing/A? generation. Furthermore, we will investigate any change in the activity, concentration or localization of appoptosin in the brain samples of AD patients and transgenic mice to establish a direct link between appoptosin and AD. Moreover, we will generate appoptosin (conditional) knockout mice to study its physiological functions. By crossing brain-specific appoptosin knockout mice with an AD mouse model, we will determine whether a deficiency of appoptosin in the brain can ameliorate AD-like pathologies and memory/behavioral deficits in the AD mice. Together, our studies will elucidate the mechanism underlying appoptosin-mediated apoptosis and demonstrate its importance in neuronal death associated with degenerative insults. The results should reveal appoptosin as a new therapeutic target for multiple diseases, such as neurodegenerative diseases and cancers. PUBLIC HEALTH RELEVANCE: Neuronal apoptosis induced by neurotoxic insults is a hallmark of many neurodegenerative diseases, including Alzheimer's disease (AD), which is primarily initiated by overproduction/aggregation of neurotoxic b-amyloid (Ab) peptides in the brain. Our identification of appoptosin as a novel pro-apoptotic protein involved in intrinsic caspase-dependent apoptosis, and demonstration of its involvement in Ab- and other neurotoxicity-induced neuronal cell death, will have a significant impact on basic research and developing new strategies for treating AD. Since apoptosis plays an important role in many other diseases, such as cancers, our studies could also reveal new targets for combating these devastating diseases.